1. Field of the Invention
The present invention relates to an optical modulator using an electro-absorption semiconductor modulator comprising an electrode of traveling wave modulator, in an optical communication system.
2. Related Arts
As to an optical communication light source in an optical communication system, there has been a continued demand for a higher speed. At present, a communication system with a transmission rate of 10 Gbits/s per channel has already been put to practical use, and a system with a transmission rate of 40 Gbits/s per channel has been developed aiming at practical use thereof. As an optical communication light source for use in such a high-speed optical communication system, an optical modulator is used. In particular, an electro-absorption optical modulator composed of a semiconductor material is a modulator which is small in size and capable of being driven at a low voltage. Furthermore, in order to broaden the band of the optical modulator, a method of adopting an electrode of traveling wave modulator has been proposed. An example of this is seen, for example, in Japanese Patent Laid-open No. 2001-326413. In this structure, the electrode is designed as a transmission line of distributed element for passing a high-frequency signal from an external modulator driver circuit and leading the signal to an external terminal resistor.
With this structure, the problem that the band is restricted by the electrostatic capacitance of an element as in the case of a lumped element can be obviated. In addition, in such a high-speed element, matching in impedance between the driver circuit and the element is required. While a method for impedance matching is not disclosed in the above-mentioned Japanese Patent Laid-open No. 2001-326413, a method of providing an impedance matching circuit composed of a coil and a capacitor between an input/output connector and an electrode of an optical device is disclosed, for example, in Japanese Patent Laid-open No. Hei 7-98442. Besides, Japanese Patent Laid-open No. Hei 9-900 discloses a method of providing a line having an impedance between the input impedance of the semiconductor chip and the output impedance of the driver circuit in on the portion between the semiconductor chip provided with the lumped element optical modulator and the driver circuit connector.
However, these impedance matching methods have the problem that the impedance matching can be achieved only for a signal at a specified impedance and it is impossible to obtain a characteristic with little signal reflection over a broad band. In addition, a modulator element having an electrode of traveling wave modulator as disclosed in the above-mentioned Japanese Patent Laid-open No. 2001-326413 has the problem that the area of the semiconductor chip would be large.
In view of the foregoing technical background, it is an object of the present invention to provide, in a small size, a semiconductor optical modulator in which reflection of an externally inputted high-frequency signal is little.
The present invention, basically, has the following constitution. Namely, an optical modulator according to the present invention has at least a transmission line, and a semiconductor chip provided at least with a semiconductor optical modulator, on a dielectric substrate. The transmission line is connected at least electrically to a first end portion of an electrode of the semiconductor optical modulator, and the semiconductor optical modulator can be driven by inputting a high-frequency signal from a modulator driver circuit to the transmission line. The characteristic impedance of at least a part of the transmission line is set to be greater than the output impedance of the modulator driver circuit.
The semiconductor optical modulator is typically exemplified by an electro-absorption semiconductor optical modulator, and a practical mode for carrying out the present invention is as follows. Namely, the traveling wave optical modulator according to the present mode comprises at least a first transmission line, a semiconductor chip provided at least with an electro-absorption semiconductor optical modulator, and a second transmission line, on a dielectric substrate. The electro-absorption semiconductor optical modulator comprises at least an electrode of traveling wave modulator, and an optical waveguide overlapping with the electrode of traveling wave modulator and having a region parallel to the electrode of traveling wave modulator. Further, the first transmission line is electrically connected to a first end portion of the electro-absorption semiconductor optical modulator via a first transmission line on the chip, whereas the second transmission line is electrically connected to a second end portion of the electro-absorption semiconductor optical modulator via a second transmission line on the chip, and the semiconductor optical modulator can be driven by inputting a high-frequency signal from a modulator driver circuit to the first transmission line. Furthermore, in the present invention, it is essential that the characteristic impedance of at least a part of the first transmission line is set to be greater than the output impedance of the modulator driver circuit.
Incidentally, in the present invention, the traveling wave optical modulator can be realized by omitting the second transmission line. The details of this will be described later.
Furthermore, it is convenient that the characteristic impedance of at least a part of the first transmission line is set to be greater than the input impedance of the electro-absorption semiconductor optical modulator from an end portion of the first transmission line.
For the optical modulator according to the present invention, a mode of integrating a semiconductor laser element directly on a substrate for the optical modulator is of use. The optical modulator is so constituted that the light emitted from the semiconductor laser is incident on an optical waveguide of the electro-absorption semiconductor optical modulator. Thus, the light emitted from the semiconductor laser is used, without introducing light from the exterior of the optical modulator.
A terminal resistor for terminating the signal having driven the optical modulator may further be integrated on the semiconductor chip.
As for the mode of mounting the semiconductor chip on a dielectric substrate, two modes can generally be adopted.
A first mode is a method of using a metallic ribbon, and a second mode is a method of using a solder layer.
Specifically, in the first mode, a semiconductor chip comprising an electro-absorption semiconductor optical modulator is mounted on a dielectric substrate so that the primary surface thereof provided with an active portion is directed up. When transmission lines on the chip which are electrically connected to both ends of an electrode of traveling wave modulator of the electro-absorption semiconductor optical modulator are made to be a third transmission line and a fourth transmission line, respectively, the first transmission line is electrically connected through a metallic ribbon to the third transmission line on the semiconductor chip on which the electro-absorption semiconductor optical modulator is mounted, whereas the second transmission line is electrically connected through a metallic ribbon to the fourth transmission line on the semiconductor chip on which the electro-absorption semiconductor optical modulator is mounted.
In the second mode, a semiconductor chip comprising an electro-absorption semiconductor optical modulator is mounted on a dielectric substrate so that the primary surface thereof provided with an active portion is directed down.
The first transmission line is electrically connected through a solder layer to the first transmission line on the semiconductor chip on which the electro-absorption semiconductor optical modulator is mounted, whereas the second transmission line is electrically connected through a solder layer to the second transmission line on the semiconductor chip on which the electro-absorption semiconductor optical modulator is mounted.
In addition, it is useful that a circuit for driving the electro-absorption semiconductor optical modulator is further mounted on the dielectric substrate. Further, a resistor for terminating a signal for driving the electro-absorption semiconductor modulator may be mounted on the dielectric substrate, and may be electrically connected to the second transmission line.